In this image, each green dot represents an individual lithium atom. The researchers used a quantum gas microscope to image the atoms, which have been cooled to a fraction of a degree above absolute zero and trapped in place using lasers. Image courtesy of Peter Brown, Princeton University.Using atoms cooled to just billionths of a degree above absolute zero, a team led by researchers at Princeton University has discovered an intriguing magnetic behavior that could help explain how high-temperature superconductivity works.
The researchers found that applying a strong magnetic field to ultracold atoms caused them to line up in an alternating pattern and lean away from each other. This behavior, which researchers call ‘canted antiferromagnetism’, is consistent with predictions from a decades-old model used to understand how superconductivity arises in certain materials. The results are published in a paper in Science.
"No one has observed this type of behavior in this system before," said Waseem Bakr, assistant professor of physics at Princeton University. "We used lasers to create artificial crystals and then explored what is happening in microscopic detail, which is something you just cannot do in an everyday material."
The experiment, conducted on a table-top in Princeton's Jadwin Hall, provides a way to investigate a model describing how quantum behaviors give rise to superconductivity, a state where current can flow without resistance, which is prized for electricity transmission and making powerful electromagnets. While the basis of conventional superconductivity is understood, researchers are still exploring the theory of high-temperature superconductivity in copper-based materials called cuprates.
Due to the complexity of cuprates, it is difficult for researchers to study them directly to find out what properties are responsible for their ability to conduct current without resistance. Instead, by building a synthetic crystal using lasers and ultracold atoms, the researchers can ask questions that are otherwise impossible to address.
Bakr and his team cooled lithium atoms to just a few ten-billionths of a degree above absolute zero, a temperature where the atoms follow the laws of quantum physics. The researchers then used lasers to create a grid to trap the ultracold atoms in place. This grid, known as an optical lattice, can be thought of as a virtual egg-tray created entirely from laser light, in which atoms can hop from one well to the next.
The team used this set-up to look at the interactions between single atoms, which can behave in a manner analogous to tiny magnets due to a quantum property called spin. The spin of each atom can orient either up or down. If two atoms land on the same site, they experience a strong repulsive interaction and spread out so that there is only one atom in each well. Atoms in neighboring wells of the egg-tray tend to have their spins aligned opposite to each other.
This effect, called antiferromagnetism, happens at very low temperatures due to the quantum nature of the cold system. When the two types of spin populations are roughly equal, the spins can rotate in any direction as long as neighboring spins remain anti-aligned.
When the researchers applied a strong magnetic field to the atoms, they saw something curious. Using a high-resolution microscope able to image individual atoms on the lattice sites, the Princeton team studied how changes in the strength of the field affected the magnetic correlations of the atoms. In the presence of a large field, the researchers found that neighboring spins remained anti-aligned but oriented themselves in a plane at a right angle to the field. Taking a closer look, they saw that the oppositely-aligned atoms canted slightly in the direction of the field, so that the magnets were still opposite facing but were not precisely aligned in the flat plane.
Spin correlations were observed last year in experiments at Harvard University, the Massachusetts Institute of Technology and Ludwig Maximilian University in Munich, Germany. But the Princeton study is the first to apply a strong field to the atoms and observe a canted antiferromagnet.
These observations were predicted by the Fermi-Hubbard model, created to explain how cuprates could be superconducting at relatively high temperatures. The Fermi-Hubbard model was developed by Philip Anderson, professor of physics, emeritus, at Princeton, who won a Nobel Prize in Physics in 1977 for his work on theoretical investigations of the electronic structure of magnetic and disordered systems.
"Understanding the Fermi-Hubbard model better could help researchers design similar materials with improved properties that can carry current without resistance," Bakr said.
The study also looked at what would happen if some of the atoms in the egg-tray were removed, introducing holes in the grid. The researchers found that when the magnetic field was applied, the response agreed with measurements performed on cuprates. "This is more evidence that the proposed Fermi-Hubbard model is probably the correct model to describe what we see in the materials," Bakr said.
This story is adapted from material from Princeton University, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.